First hard look at Zika pregnancies finds nearly half result in miscarriage or birth defects

New data from Rio de Janeiro, Brazil, suggest that nearly half of women infected with Zika virus during pregnancy experience a serious complication, whether a miscarriage or significant birth defect, in their baby. The data are the first to quantify the risks to women infected at different times during pregnancy, and they seem to confirm that they are highest early in pregnancy. But the study also finds significant problems among women infected just a few weeks before giving birth. Microcephaly is the best known birth defect, but those dramatic cases represent only a small proportion of children damaged by the virus.

The study is relatively small—researchers in Brazil followed just 125 infected women—but it is the first to report pregnancy outcomes from women conclusively known to have the virus. Only such cohort studies are able to determine how high the risk of birth defects and other problems is for infected pregnant women and their babies, says Maria Van Kerkhove, an epidemiologist at the Pasteur Institute in Paris who wasn’t involved in the study. The results are crucial for being able to counsel expectant mothers, she says.

The data, presented online today in The New England Journal of Medicine, are a follow-up to a preliminary report published in March. That study found that among women infected in the third trimester, 29% developed complications that affected their babies. The new data describe outcomes for women infected earlier in pregnancy, who in March had not yet given birth.

Patricia Brasil, an infectious disease researcher at the Oswaldo Cruz Foundation in Rio de Janeiro, and her colleagues began by studying 345 pregnant women who experienced a rash—a telltale symptom of Zika infection—between September 2015 and May. Of that group,182 tested positive for Zika virus. By July, the team had data on 125 Zika-infected women and their babies and 61 women who had given birth after the rash but had not tested positive for Zika.

The results were sobering. Among women infected in the first trimester, 55% experienced what the researchers called “adverse outcomes.” That included miscarriage, calcifications in a baby’s brain (a sign of abnormal brain development), babies being born overall much smaller than normal, and brain hemorrhages. Among women infected in the second trimester, 51% experienced such adverse outcomes. Overall, such serious problems affected 58 of the 125 infected women, or 46%. Among women who did not test positive for Zika virus, seven, or 11.5%, had adverse outcomes. The updated paper describes four cases of microcephaly among the 125 infected women, or 3.4%.

And 46% may not be the whole story, says Karin Nielsen-Saines of the David Geffen School of Medicine at the University of California, Los Angeles, who helped to coordinate the study. The data published this week include observations during babies’ first 3 months of life. Other researchers have reported that Zika-affected babies that seem normal at birth can still have brain damage that becomes apparent later. The paper notes that several infants who had no obvious problems at first had abnormal MRI brain scans and should be followed closely. “Some babies are damaged invisibly at birth, and as they’re supposed to develop the problems will emerge,” Nielsen-Saines says. The team has tried to collect brain imaging of as many babies as it can.

The news is grim for regions affected by the virus, she concludes. “It’s sobering. The chances of someone having an abnormal outcome are high.”